chore: set default cli help page to start instead of index

Due to changes in the previous commit, `tns -h` is not working as CLI is trying to open the `index.md` file, which does not exist anymore. Change CLI's code to look for `start.md` instead.

Author: rosen-vladimirov

lib/common/services/help-service.ts

@@ -195,7 +195,7 @@ export class HelpService implements IHelpService {
 			await this.throwMissingCommandError(commandData);
 		}
 
-		return (commandName && commandName.replace(/\|/g, "-")) || "index";
+		return (commandName && commandName.replace(/\|/g, "-")) || "start";
 	}
 
 	private async throwMissingCommandError(commandData: ICommandData): Promise<void> {

lib/common/test/unit-tests/services/help-service.ts

@@ -422,7 +422,7 @@ and another one`
 			it("shows index help when command is not specified", async () => {
 				const injector = createTestInjector();
 				injector.register("fs", {
-					enumerateFilesInDirectorySync: (path: string) => ["index.md"],
+					enumerateFilesInDirectorySync: (path: string) => ["start.md"],
 					readText: () => "index data is read"
 				});